Over the past few years, the use of fiber optic networks in communication systems has increased dramatically. Such fiber optic networks are commonly employed, for example, in long distance telecommunication systems, cable television systems, and Internet cable systems. In the future, the use of fiber optic networks will become even more prevalent as a preferred medium for transferring information as the marketplace for wide-bandwidth services matures. For instance, such services may include video-on-demand, interactive television and games, image networking, and video conferencing.
As the demand for fiber optic networks increases, the development of new supporting technologies and the refinement of existing technologies is required for the implementation of the above-identified services. One key for any such fiber optic network is the ability to multiplex and demultiplex optical signals. One preferred optical device for performing such functions is a wavelength division multiplexer (WDM).
A WDM is a device with multiple optical paths, each of which exhibits a particular wavelength passband. Each passband permits passage of one or more particular wavelengths (i.e., a Achannel@) along the respective optical path, to the substantial exclusion of others. Thus, the WDM can be used to a divide multichannel optical signal into specific wavelength channels, or to combine various channels on respective optical paths into one multichannel optical signal on one optical path.
Three basic classes of WDMs are commonly used, and are classified as coarse, intermediate, and dense. Coarse WDMs are configured for dividing and combining two channels that are spaced relatively far apart, e.g., a 1310/1550 nanometers (nm) WDM used to separate wavelength channels with a 100 nm bandwidth centered around 1310 nm and 1550 nm. Intermediate WDMs are configured for dividing and combining two to three channels that are spaced closer than those of the course WDMs, e.g., a 1540/1560 nm WDM used to space two channels approximately 20 nm apart in the 1550 nm wavelength band. Lastly, and subject of the present invention, dense WDMs (also referred to as DWDMs) are configured for dividing and combining four or more channels that are very closely spaced, e.g., 32 channels having a spacing of less than 1.0 nm.
DWDM transmitters in closely spaced DWDM transmission systems require accurate wavelength setting and stabilization. In many cases, to ensure system reliability, active wavelength monitoring and stabilization techniques are used to independently stabilize each transmitter in the DWDM array. Unfortunately, previously available wavelength monitoring and stabilization techniques are often complex, expensive, difficult to implement, and have limited effectiveness.